What is "OpenGL" and why did a player get banned for it?

OpenGL, or Open Graphics Library is a programming interface that games use nowadays.

It is essentially a library of code that is used by developers to create software utilizing graphics, which nowadays, is essentially nearly everything.
Examples of other Graphics Libraries are .NET's System.Drawing namespace, Qt and DirectX; the one used and made by Microsoft.

As for what it is programmatically:

The OpenGL specification describes an abstract API for drawing 2D and 3D graphics. Although it is possible for the API to be implemented entirely in software, it is designed to be implemented mostly or entirely in hardware.

Source

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As for the cause of the ban, one of the most common reasons is because the user is using a modified opengl32.dll.

However, since your log is very vague (as pointed out by user2357112), the real reason: We'll never know.

What can this opengl32.dll can do to a server?

Nothing.

The graphics library is only used on the client-side so it can render (show) the game on-screen. Without it, nothing really happens on the screen. The best you'll get for a game without it (a graphics library) is a command prompt window.

What you can do, however... Is create a modified version of the OpenGL graphics library which hooks into the game's core code to allow changes to the renderer, such as wall hacks or x-ray, which are in the eyes of VAC - are cheats.

These cheats don't affect the server, but they do give the player an advantage since they can see things that other players cannot see.

Do other games' Wallhack use this kind of thing too?

Well, it depends on the game. Wallhacking involves the removal of textures on a geometry to let you see through it.

There are many ways to do this, including:

• Swapping the texture on a polygon with nothing (can be done many ways; file swapping, memory hacking, etc.)
• Changing the opacity of the polygon's texture (can be done many ways; file swapping, memory hacking, etc.)
• Simply not rendering the polygon (can only be done with injected code/modded libraries/game binaries)

While others used a rather vague explanation of what is the file itself, the modified opengl library would give the player a big advantage over the others such as seeing through walls, computer controlled aim, and so on. In other words he was cheating and got banned for that.